There are numerous systems available for providing a double or dual-containment piping assembly including inner or primary pipes contained within outer or secondary containment pipes to deliver dangerous or hazardous fluid within the inner pipes. In the event of a leak or emission of the fluid or vapor from the inner pipes, the leaking substance is intended to be safely contained within the outer pipes. Historical applications for such double-containment systems are found in the nuclear, gas and petroleum refining, and chemical processing industries. It is also known to provide certain types of detectors and/or drainage devices in the annulus between the inner and outer pipes in the event that there is leakage to detect and/or collect any such leakage.
Differential thermal expansion in double-containment systems occurs when the inner and outer pipes expand or contract to different degrees, at different rates, or both. Almost all chemical and petroleum double-containment systems are subject to changes in temperature during operation, or differences in operating temperature between the inner and outer piping components. Such temperature changes can result in relatively substantial expansion and/or contraction of the inner piping components relative to the outer piping components, or vice-versa. For example, in underground double-containment systems, there is typically relatively cool soil or earth surrounding the outer piping components, and a relatively hot fluid flowing through the inner piping components, increasing the temperature of the inner piping components relative to the outer piping components, and in turn causing differential thermal expansion of the inner piping toward the outer piping. Another typical situation occurs with above-ground double-containment systems, when such systems are initially installed during relatively hot weather, and a substantially constant temperature fluid is pumped through the inner piping components, and the systems are subsequently subjected to ambient cooling. In this situation, the outer pipes contract relative to the inner pipes, and move toward the inner pipes.
When the inner piping components expand or contract relative to the outer piping components, and if the inner piping components are installed in an axially unrestrained manner, the deflection of the primary piping due to the growth or contraction of the inner piping accumulates at the elbow sections of the inner piping. In this case, the inner elbow sections are subjected to bending and/or torsional movements. Although elbow fittings by their nature are capable of greater flexibility than comparable straight sections of pipe, when elbow sections are subject to bending and/or torsional movements, stresses are intensified, and in some instances, this intensification in stress can lead to failure. There are piping design codes setting forth the maximum allowable stress, depending upon the material, wall thickness, etc., of elbow components.
In the United States and Canada, and many other parts of the world, the code which governs the design and implementation of chemical and petroleum piping systems is the ANSI/ASME B31.3 Chemical Plant and Petroleum Refinery Piping Code, which presents simplified methods for determining bending stresses of elbows in chemical piping systems due to expansion and contraction. Appendix D of this Code presents equations for a flexibility factor (k), in-plane and out-of-plane stress intensification factors, and the flexibility characteristics of many piping components, including elbows.
Based on these established equations, all other variables being equal, the greater the centerline-to-end radius (R) (or the centerline radius) of an elbow section in comparison to the cross-sectional radius (r) of the elbow section, the higher the value of the flexibility characteristic (R), and thus the lower is the resultant stress intensification factors (.sup.i o and .sup.i i). Accordingly, the lower the values of the stress intensification factors, the lower is the resultant bending stress at the elbow. In a double-containment piping assembly, therefore, if the elbow fitting is free to bend, and the piping adjacent to the elbow fitting is free to bend, the greater the centerline-to-end radius of each elbow section (up to reasonable limits), the lesser is the stress that is developed within the elbow fitting.
In many double-containment piping systems to date, however, the inner piping components are not permitted to bend or otherwise move either laterally or axially relative to the outer piping components, particularly in the area of the elbow fittings. In other known double-containment piping systems, the inner piping components may be able to move relative to the outer piping components, but only within narrow limits, and once these narrow limits are exceeded, the inner piping components come into contact with the outer piping components. In either case, the elbow fittings are not allowed to fully bend or flex in response to differential thermal expansion or contraction; instead, the elbow fittings essentially behave as internal anchors, and become points of restraint. In these double-containment systems, there is no means for accommodating or alleviating the differential thermal expansion and/or contraction of the inner and outer piping components relative to each other, and thus such systems operate as restrained systems, developing large axial stresses, which can lead to failure, and leakage of hazardous fluids and vapors.
A typical practise in the double-containment piping industry has been to use standard off-the-shelf elbow sections for single-wall piping applications to form the elbow fittings for double-containment piping systems. Elbow fittings produced commercially throughout the world are standardized in terms of radii. Historically, elbow fittings have primarily been produced using two standard conventions for defining the centerline radius of curvature, regardless of the material of construction of the pipe, or the diameter or wall thickness of the pipe. These two standard conventions are the "short-radius" elbow fitting and the "long-radius" elbow fitting. The short-radius elbow fitting is defined as having a centerline radius of curvature which is approximately equal to the nominal diameter of a corresponding straight section of pipe. A long-radius elbow fitting is defined as having a centerline radius-of-curvature which is approximately equal to 1.5 times the diameter of the corresponding straight section of pipe.
Another type of standard elbow fitting somewhat common for sanitary applications, referred to as the "long-sweep sanitary elbow", has a centerline radius of curvature approximately equal to 1 to 1.5 times the nominal diameter of a corresponding straight section of pipe.
The standard practice in the double-containment piping industry has been to use commercially available short-radius/short-radius combinations of such inner and outer elbow fittings. In this case, the centerline radius of curvature of the inner elbow section is always less than the centerline radius of curvature of the outer elbow section. Because the corresponding straight sections of pipe are concentrically mounted with respect to each other, the inner elbow section is not concentrically mounted within the outer elbow fitting, but rather is offset so that the spacing is more narrow between the larger-radius surfaces of the inner and outer elbow sections than it is between the smaller-radius surfaces of the inner and outer elbow sections.
As a result, when there is differential thermal expansion or contraction between the inner and outer piping components, the inner and outer elbow sections frequently contact each other at the point where there is the least amount of space between the fittings, thus causing a high localized stress, which can in turn cause one or both fittings to fracture. This is the source of numerous premature failures in known double-containment piping systems.
Another drawback of these prior double-containment piping systems employing standard fittings, is that the centerline radius of the inner elbow section is less than the centerline radius of the outer elbow section, thus typically creating a higher level of stress within the inner elbow section than is desired during operation when the elbow section is subjected to bending or torsional movements, which can in turn lead to premature failure.
It is an object of the present invention to overcome the drawbacks and disadvantages of known double-containment piping systems, particularly the elbow fittings of known double-containment piping systems.